Due to their small size and low operating voltages, integrated circuits or chips are susceptible to damage from electrostatic discharge (“ESD”). For example, one model for ESD considered in the industry is the H.B.M. (human body model or “HBM”). It is an industry standard to protect chips to withstand an HBM discharge of 2,000 volts with current transients corresponding to about 1.3 amps. Though an HBM event is short in duration, such an event if coupled directly to an input or output pin on a chip without ESD protection, will almost certainly destroy the circuitry of the chip. ESD events which are transmitted to a chip frequently travel through the chip on the power supply rail or simply power rails (i.e., power or voltage supply rail and ground rail) of the power supplies internal to the chip. Accordingly, ESD events will typically cause large voltage and current transients across the power rails of the chip. Thus, circuits which protect against ESD damage in a chip typically allow the ESD event to be discharged from the power supply rail of the chip to a ground rail. Such an ESD protection circuit is typically called a “power clamp.”
Related art power clamps include diode-based circuits. In a diode-based power clamp, diodes having a prescribed turn-on voltage are connected between a power rail and a ground rail. When the voltage potential between the power rails exceeds the prescribed voltage, the diodes turn on and short the power rails to one another. Diode-based power clamps have certain disadvantages, including a relatively high on-resistance. Additionally, the turn-on voltage may be relatively high to allow the circuit to operate at its normal voltage, multiple diodes must be connected in series, thereby increasing the on-resistance of the power clamp. Power clamps having a high internal on-resistance are less effective because voltage between a power rail and a ground rail must be kept low during the ESD event, thereby precluding power clamps having high on-resistances. In addition, grounded-gate nFETs and silicon controlled rectifiers (“SCR”) are also used for ESD protection between a power rail and a ground rail. The disadvantages of these devices are very high trigger voltages and very high on-resistances, respectively.
Some chip designs include multiple power supplies, where at least one of the power supplies operates at a voltage higher than the native voltage of the devices in the technology. As such, the power clamp must be able to withstand the operating voltage of the higher voltage power supplies, while being fabricated for the native technology voltages of the chip.
One type of power clamp relies on a single nFET between a power rail and a ground rail where the nFET is in the off-state during the normal operation of the chip, but turns on during the transient pulse of an ESD event. Once in the on-state, the nFET provides a low-resistance path to the anomalous voltage spike from the power supply rail to the ground rail.
More specifically, a typical power clamp uses a single nFET positioned between a voltage or power supply rail and ground rail. A typical power clamp also includes a RC circuit having a resistor and a capacitor connected in series with one another between the power supply rail and the ground rail. The RC circuit is connected to the gate of the nFET between the capacitor and the resistor.
In operation, the nFET of the typical power supply is biased into an off-state by the RC circuit during normal operation. When a voltage spike or transient occurs on the power supply rail of sufficient amplitude and frequency to qualify as an ESD event, a portion of the ESD event is coupled to the gate of the nFET through the RC circuit. When the ESD event occurs, the RC circuit biases the nFET into the on-state, thereby shorting the power supply rail to the ground.
The values of the resistor and the capacitor of the RC circuit are commonly chosen to have an RC time constant of about 1 microsecond. The RC circuit is configured to function as a high pass filter which will trigger the nFET into the on-state when an ESD event occurs on the power supply rail. The capacitor and the resistor together form a trigger circuit. The trigger circuit may be considered to function similarly to a high-pass filter and will allow a high frequency voltage event, such as an ESD event, to be transmitted to the gate of the nFET to switch it to the on-state.
nFETs used for power clamps should have low resistance to effectively shunt power to ground under ESD conditions. Additionally, the operating voltage of the power supply incorporating the power clamp cannot be above the maximum allowed nFET voltage, otherwise, the nFET will be damaged during normal operation.